Sim card slow to detect network coverage

My Oppo f11 pro once got into water. It worked well after that for a day but my camera is wet. but on the day after suddenly my sim card can't be read. There's no service and no network coverage. I can't make and accept calls and internet data won't work either. I went to the oppo service center to fix this, they dry my phone and my camera works perfectly again. But they told my simcard encounter problems because the main board is damaged. And it is very costly to replace the mainboard. But other fucntions works perfectly well. Now my sim card can be read, but very slowly like for 10 minutes to detect it? Even so I still can’t use internet data and make and accept calls. What should I do?